Conventional machines have been designed for voltages in the range 15-30 kV, and 30 kV has normally been considered to be an upper limit. In the case of generators this normally means that a generator must be connected to the power network via a step-up transformer. The voltage level of the power network can be in the range of approximately 130-400 kV.
The present invention is intended to be used with high voltages. In this specification the term "High voltages" means electric voltages exceding 10 kV, Typically, an operating range for a machine wound in accordance with the invention and using a cable forerunner according to the invention may be voltages from 36 kV up to 800 kV.
By using high-voltage insulated electric conductors in the following termed high-voltage cables, with solid insulation similar to that used in cables for transmitting electric power (e.g. XLPE cables) in a rotating electric machine, the voltage of the machine can be increased to such levels that it can be connected directly to the power network without intermediate transformers. The conventional transformer can thus be eliminated. The cable is provided with an outer semi-conducting layer with the help of which its outer potential is defined. The high voltage cables thus enclose the electrical field within the windings. Such an insulated conductor or cable is flexible and it is of a kind which is described more in detail in the PCT applications SE97/00874 and SE97/00875. Additional descriptions of the concerned insulated conductor or cable can be found in the PCT applications SE 97/009001, SE 97/00902 and SE97/00903.
This concept generally implies that the slots in which the cables are arranged in the stator to be deeper than conventional technology (thicker insulation due to higher voltage and more turns in the winding). This entails new problems in winding the high-voltage cable in such machines.
Many different methods are available for winding the stator in a rotating electric machine with conventional windings. However, all these methods presume that the windings are stiff and can be inserted without being wound. Further, conventional windings are normally divided into short parts to fit the slots of the stator. These methods cannot be used in the present invention, where the windings consist of high voltage cables of substantial lengths, which have large diameter, high weight per unit length and high flexural rigidity. Further, during the winding process, the outer semiconducting layer of the cable must not be damaged. The difficulties in winding such a cable are thus considerable.